When someone gets pregnant unexpectedly/at a very inconvenient time in their life and never gets an abortion.
When someone who expresses a lack of interest in having kids at the last moment does or is talked into it to 'tie everything up' (sometimes it doesn't happen at the last moment but it always seems rushed/fake on screen (like when they shove it into a single episode wut,) examples include:
There's a tendency to make the least maternal female character have kids and for an entire plot line to be created around her doing so, and getting pregnant while her more feminine friend struggles and wants kids eg:
edit: another thing that's common on TV shows is characters who want children always get them eventually. Whether it's biological or through adoption. Characters who don't want kids have kids, characters who don't want kids sometimes don't have kids, characters who want kids have kids. Never seen a storyline where someone wants a kid and it just never happens.
When a cast all end up with each other in the end.
Hooking up characters with no chemistry together due to probably some self insert desires of the writers. Key example from what I remember was Penny and Leonard on The Big Bang Theory legit would have made more sense for her to hook up with Sheldon if anyone. Not that any relationship on that show could have been healthy though.